In the canned motor pump, a portion of a pump-treating liquid is usually withdrawn from a delivery port of a pump section and introduced into a canned motor. Thereafter, the liquid portion flows over a surface of a bearing for a rotor shaft and then between a stator and a rotor. The liquid portion is then recycled to a pump system. In this case, an amount of the lubricating and cooling liquid for flowing over the bearing surface and the motor-cooling surface should be maintained at a neccessary and proper level for reducing an unwanted leakage and improving the efficiency of the pump.
From this point of view, conventional canned motor pumps have been provided at inner circumferences of their front and rear bearings with helical or axial grooves.
However, the grooves in the front and rear bearings of the conventional canned motor pumps are identical in their shapes and hence in their flow resistance. For this reason, the bearing especially on the side for introducing the treating liquid is subjected to a large pressure difference before and after the bearing, thereby to increase the unwanted excessive amount of the flowing liquid (the leakage) and to lower the designed pump efficiency. Particularly in the canned motor pump of a reverse circulation system in which a portion of the treating liquid is introduced through a junction between the pump section and the motor section into the canned motor for lubricating its front bearing, then is passed between the stator and the rotor for cooling the motor and subsequently lubricating the rear bearing, and thereafter is recycled to a suction port of the pump, the pressure drop as previously described is generated in the front bearing, thereby to increase the unwanted flow of the liquid and to lower the pump efficiency considerably.
Accordingly, an object of the invention is to provide a canned motor pump in which a flow resistance to a lubricating surface may be changed for front and rear bearings located at different pressure conditions in a motor section for minimizing the necessary amount of a lubricating liquid, thereby to reduce leakage in a pump section and to improve the pump efficiency.